Ayep, off by (2*pi)^2.
Correction:
circumfrence / day = c
circumfrence = radius * 2 * pi
radius
* 2 * pi = c* 1 day
radius = c * day
/ 2 / pi
Using google as a calculator:
http://www.google.ca/search?q=c*60*6...conds%2Fpi%2F2
I get:
4.12244222 × 10^12 meters
Or, 646 343.302 larger than the real earth.